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+ ESHJJIOA INVENTOR Arthur W Vance 'QNX "D BY a) HTTOR EY July 19, 1938.

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BYQ] l Patented July 19, 1938 UNITED STATES PATENT OFFlCE ELECTRICAL APPARATUS of Delaware Application March 31,

14 Claims.

My invention relates to electrical signalling apparatus and particularly to direct-current ampliiiers.

In many electrical circuits it has been found 5 desirable to utilize direct-current amplifiers rather than ampliiiers which can not transmit a direct-current component. Direct-current ampliiiers, however, have a serious defect in that they have a tendency to drift, That is, with no signal on the amplier, the output current will gradually increase or gradually decrease over a period of several minutes.

In high gain direct-current amplifiers, such as are especially desirable for use in television r transmitters, the drift is so serious that heretofore it has been practically impossible to use direct-current ampliers in television transmitters. Their use in such transmitters is desirable in order that the direct-current component of the picture background may be transmitted.

It is an object of my invention to provide an improved direct-current amplifier and an improved method of operating the same. n

More specifically, it is an object of my invention to provide a direct-current amplifier which is stable in operation and which does not drift.

It is a further object of my invention to provide an improved television or facsimile system.

It is a still further object of my invention to 30 provide an improved television or facsimile transmitter which includes a high gain direct-current amplifier.

In practicing my invention, I provide a directcurrent amplier with a closed loop circuit so 3.3 connected that energy may be fed from the output circuit of the amplifier to its input circuit in the proper phase to oppose any change in voltage or current at the output circuit. The signals to be ampliiied, such as picture signals, are impressed upon the ampliiier input circuit periodically whereby there are intervals during which no signal is impressed thereon.

The loop circuit is so designed that it is ineffective during the period a signal is impressed upon the amplifier, the signal being given a positive polarity to accomplish this. During the interval of no signal, an impulse of negative polarity, such as a synchronizing impulse, is impressed upon the amplier. This negative impulse causes a iiow of current through the loop circuit to charge a condenser and control a bias voltage in the amplifier during the succeeding signal period, this flow of current having a magnitude which depends upon any drift that has occurred during the preceding signalperiod. In

1934, Serial No. 718,354

this way the degenerative loop circuit prevents any appreciable drifting of the amplifier even though it has a very high gain.

Other objects, features and advantages of my invention will appear from the following de- *f scription taken in connection with the accompanying drawings, in which:

Figure 1 is a block diagram of a television system embodying my invention;

Fig. 2 is a fragmentary view of the scanning disc shown in Figure 1;

Fig. 3 is a curve showing thecharacter of the signals impressed upon the main direct-current ampliiier of the transmitter;

Fig. 4 is a circuit diagram of the apparatus f shown in Figure l;

Fig. 5 is a set of curves showing the operation of my improved direct-current amplifier;

iFig. 6 is a block diagram showing a modification of the transmitter circuit illustrated in Referring to Figure 1, a television transmitter is shown at theleft-hand side of the drawing while a television receiver is shown at the righthand side of the drawing, the transmitter and receiver being connected by a transmission channel as indicated by the dotted lines l and 3. The channel has been exemplified as -conductive merely for purpose of convenience, it being of course understood that my invention is applicable to the ampliiiers utilized in a radio transmitter orradio receiver. In the particular embodiment illustrated, the transmitter is shown set up for the transmission of the pictures on a motion picture film Yindicated at 5.

The iilm 5 is scanned by means of a scanning disc Iv driven by a motor 9, the scanning disc preferably being ofv the type described and claimed inmy co-pending application Serial No, 544,959, filed June 17, 1931, and assigned to the same assignee as this application. As indicated in Fig. 2, a disc of this type has a `row of scanning holes ll, and a row of` synchronizing holes i3 arranged on a circle near the circumference of the disc. v

Light for scanning the picture is supplied from I e Y a suitable source l5 and passed through an optical system indicated at |1, the light passing through the scanning holes of the disc and through the film 5 onto a photoelectric cell I9. The photoelectric cell I9 is connected through a battery 2| to the input circuit of a direct-current amplifier 23.

The output circuit of the amplifier 23 isconnected by means of a coupling resistor 25 and inductance coil 21 to the input circuit of the main direct-current amplifier 29 which has a high gain. The coil 21 is employed for improving the high frequency response of the system. The coupling circuit between the ampliers 23 and 29 also includes a condenser 3|, which is utilized, as explained hereinafter, in combination with a balancing circuit for preventing drifting of the direct-current amplifier 29.y

The output circuit of the amplifier may be connected to the transmission line I-3, as indicated on the drawings, or it may be connected to a radio transmitter for transmitting the picture signal over a radio channel in accordance with well known practice.

In order to prevent drifting of the high gain amplifier 29, a balancing network indicated at 33 is provided, its input circuit being connected to the output circuit of the ampliiier 29 and its output circuit, exemplified by the variable resistor shown in dotted lines, being connected across the condenser 3|.

A'suitable light source 35 and optical system 31 are provided for'passing light through the'synchronizing openings I3 in the disc 1 and upon a photo-electric cell 39.` The photo-electric cell 39 is connected through a battery 4| to the input circuit 'of a synchronizing amplifier 43. The output circuit of the synchronizing amplifier 43 is connected to the input circuit of the directcurrent amplifier 23 through a conductor 45 and a connection which includes a condenser Hand a resistor 49. n v' As explained in my above mentioned co-pending application, the synchronizing amplifier 43 is so designed that'` it supplies synchronizing pulses to the direct-currentampliiier 23 which have a polarity opposite to the polarity of the picture signals, theV scanning disc being provided'with masks so positioned that the synchronizing impulses occur during the intervals that no picture signals are being transmitted.v f The character of the signal applied to the input circuit of the Yamplifier 23 is lshown in Fig. 3. It will be seen that each scanning line of the picture is representedbyvpositive voltage signal waves 5|, these signal wavesbeing separated by short intervals. During these intervals synchronizing impulses 53 which have a negative polarity are impressed upon the amplifier 23. In accordance withmy invention, these negative impulses are utilized both for synchronizing the television vapparatus and for controlling the balancing circuit 33 whereby the high gain direct-current *amplier 29 is stabilized, or they may be utilized solely for stabilizing the amplifier. y

The receiving apparatus shown in Fig. 1 cornprisesa direct-current amplifier 55 which amplies the picture signal and impresses it upon the control grid 51 and cathode 59 of a cathode ray receiving tube 6|. The cathode ray tube is provided with deflecting lcoils 63 and 65 connected to oscillators indicatedfat 61 and 68, respectively, which provide saw-tooth waves for deflecting the cathode ray beam both `in a horizontal and in a vertical direction. The deiiecting oscillators 61 and 68 are maintained in synchronism with the transmitting apparatus by means of a synchronizing selector 10 connected to the transmission line |-3 for supplying synchronizing pulses to the deecting oscillators. A separating circuit 66 is included in the circuit between the selector 69 and the oscillators 61 and 38 for the purpose of separating the horizontal and vertical synchronizing impulses. This method of maintaining synchronism between the transmitting and receiving apparatus is described and claimed in my above-mentioned co-pending application and 1n application Serial No. 565,953, filed September 30, 1931, in the name of R. D. Kell, and assigned to the same assignee as this application.

'I'he invention will be more clearly understood by referring to the circuit diagram in Fig. 4 in which parts similar to those in Fig. 1 are indicated by like reference numerals. The directcurrent amplifier 23 comprises an electric discharge tube 1| having a cathode 13, a control grid 15, a screen grid 11, and an anode 19. The control grid 15 is maintained at the proper negative bias with respect to the cathode 13 by means of a biasing battery 8| and av resistor 83. The photo-electric cell I9 comprises a cathode 85 and an anode 81, the cathode being connected to the control grid 15 and the anode 81 being connected throughthe battery 2| to the cathode 13.

The anode 19 of the tube 1| is maintained at the proper positive potential by means of a connection through the plate resistor 25 and coil 21 to the positive terminal of a plate battery 89. The screen grid `11 is connected to a suitable point on battery 89.

The main direct-current amplifier 29, which has a gain so high that precautions must be taken to avoid drifting, comprises an electric discharge tube 9| having a cathode 93, a control grid T', `a screen grid 91,v and an anode 99, and a second electricv discharge Atube |0I, preferably of thepentode type, having a cathode |03, a control grid |05, a suppressor grid |01, a screen grid |09,and an anode The control grid 95 of the tube 9| is connected directly to the anode 19 of the preceding tube 1|.A The cathode 93 is connected to the lower terminal of thecoil 21 through the condenser 3| anduthrough the balancing circuit 33 in a manner described hereinafter.

The anode 99 is maintained at avpositive potential bya connection through a plate resistor ||3 and a coil ||5 to the positive terminal of a plate battery ||1, the screen grid 91 being connected to afsutable positive point thereon.

The control grid |05 of the pentode |0| is connected" directly to the anode 99 of the tube 9|. The cathode |03 is connected to a suitable point on the plate battery ||1 for biasing the control grid |05 to a proper value. The anode of the pentode4 `|0| is connected through a plate resistor I9 to the positive terminal of a plate battery |2I, while the suppressor grid |01 and the screen grid |99 are connected to suitable points thereon. i

The balancing circuit comprises an electric discharge tube |23 having a cathode |25, a ccntrol grid |21, a screen grid |29, and an anode |3I. The control grid |21 is connected through `a biasing battery |33 to the anode of the directcurrent amplifier 29. The cathode |25 is connected through a conductor |35 to the cathode 93 of the amplifier ltube 9|, whereby the connection of input circuit of the balancer tube |23 to the output circuit of the direct-current amplier 29 is completed.

The anode |3| is connected through a plate resistor |37 to the positive terminal of a plate battery |39, while the screen grid |29 is connected to a suitable point thereon.

The output circuit of the balancer tube |23 is connected across the condenser 3|,l this connection being through the conductor |35 and through a biasing battery |42 which has its positive terminal connected to the anode |3| and its negative terminal connected through a conductor |43 to the grid side of the condenser 3|.

The biasing battery |33 is preferably given a value such that the balancer tube |23 is biased slightly beyond the cut-oi point. Because of this, the picture signals, which have a negative polarity at the point where they are impressed upon the balancer tube, have no effect thereon, while the synchronizing impulses, which have a positive polarity at this point in the circuit, p-roduce a pulse of current in the output circuit oi the balancer tube for charging the condenser 3|.

The effect of these current pulses in the output circuit will be better understood by tracing the biasing circuit for the tube 9| of the amplier 29. This circuit may be traced from the cathode 93 through the conductor |35, the plate battery |39, the plate resistor |37, the biasing battery Mi, the conductor |43, and through the coil 27 and plate resistor 25 to the control grid 95. It will be noted that the voltage between the cathode |25 and the plate 73| has a polarity opposed to that of the biasing battery MI. Therefore, if the positive voltage on the plate |3| decreases, the negative bias applied to the control grid 95 is increased. It follows, therefore, that the appearance of a pulse of current in the plate circuit of the balancer tube |23 tends to increase the bias on the control grid 95, since such a pulse of current lowers the voltage on the anode |3| due to the IR, drop in the resistor.

Because of the condenser 3|, however, the change in bias on the control grid 95 varies gradually with changes in the magnitude of the plate current pulses, these pulses charging up the condenser 3| to a value depending upon their magnitude. The condenser 3| and the plate resistor |37, which is effectively in shunt to the condenser, have sufliciently large values to integrate the plate current pulses whereby a bias is applied to` the input circuit of the high gain amplifier 29 which varies gradually in accordance with any drifting of the amplier.

The synchronizing amplier |53, which supplies the synchronizing or control pulses during the intervals of no picture signals, comprises an electric discharge tube |45 having a cathode |57, a control grid |49, a screen grid |5|, and an anode |53. The control grid |49 is maintained at a suitable bias by means of a connection to the cathode |47 through a resistor |55 and a biasing battery |57. The cathode |59 of the photoelectric cell 39 is connected directly to the control grid |49 while the anode IBI is connected through the battery 4| to the lower terminal cf the resistor |55.

The anode |53 is connected through a plate resistor |63 to the positive terminal of a plate battery E55, while the screen grid |5| is connected to a suitable point thereon. The output circuit of the synchronizing amplier 43 is connected to the input circuit of the rst direct-current amplifier 23 by means of the conductors 45 and |35 and condenser 3| and by means of the connection from the anode |53 to the control grid 75 of the amplier tube 7| through the condenser 47 and resistor 49.

It will be evident that light striking the photoelectric cell 39 of the synchronizing circuit and the light striking the photoelectric cell I9 of the picture amplier cause pulses of current at the input of the main direct-current amplifier 29 which are of opposite polarity, since the synchronizing light pulse is amplified through two amplifiers and the picture impulse is amplied through only one amplier.

At the receiving system, the direct current amplifier comprises an electric discharge tube |67 having a cathode |69, a control grid |7|, a screen grid |73, a suppressor grid |75, and an `anode |77. The control grid |7| is connected through the transmission line conductor to the output circuit of the -direct-current amplier 29 of the transmitter. The cathode |69, which is heated through a transformer winding |79, is connected through the other transmission line conductor 3 to the proper point on the plate battery |2| for maintaining the control grid |7| at a suitable bias potential.

The anode |77 is connected through a plate resistor |8| to the positive terminal of a plate battery |83 while the screen grid |73 is connected to a suitable positive point thereon. The control grid 57 of the cathode ray tube 6| is connected through a variable tap to a point on the plate resistor |8|, while the cathode 59 is connected to a suitable point on the plate battery |83 for biasing the control grid 57 to the proper value.

The synchronizing selector 79 comprises an electric ydischarge tube 69 having a cathode |85, a control grid |87, a screen grid |89, and an anode |9|. The control grid |87 and cathode |35 are connected to the transmission line conductors and 3 respectively, the cathode connection lincluding a biasing battery |93 having the proper value for biasing the tube 69 beyondv the cut-off point.

The anode |9| is connected through a plate resistor |95 to the positive terminal of a plate battery |97, while the screen grid |89 is connected to a suitable point thereon. The output circuit of the selector tube 59 is connected to the separating circuit 55 by means of a connection through a couplingv condenser |99 and by means of a connection through ground indicated at 29|.

It will be seen that since the synchronizing selector tube 59 is biased beyond the cut-off point, it will not be aiTected by vpicture signals, while the synchronizing impulses, which have a positive polarity at this point in the circuit, will be impressed upon the deecting oscillators 67 and 63.

Referring again to the operation of the high gain direct-current amplier 29 and its associated balancing circuit 33, the action of the balancer tube |23 and the biasing condenser 3| is shown by'the curve in Figure 5. In this gure the curve 293 represents the plate current of the balancer tube |23 plotted against voltage applied to its control grid. The curves for the picture and synchronizing signals 5| and 53, respectively, are drawn along a time axis which passes through the value of grid voltage on the abscissa which is equal to the bias voltage on the balancer tube |23. It will be noted that this bias voltage is greater than the value required for complete cut-oi of the tube. The curves drawn in solid lines represent a condition of the amplifier 29 where there is no tendency to drift.

Since the balancer tube |23 is biased beyond the cut-off point, the picture signals have no effect upon the balancer circuit. The synchronizing impulses, however, have the proper polarity and magnitude to cause plate current to fiow in the balancer tube each time a synchronizing pulse occurs. These pulses of plate current are represented by the solid line rectangles 205. They are integrated by means of the condenser 3| to a substantially constant current having a value indicated by the dotted line 201. Thus it will be seen that the bias on the tube 93 of the high gain amplifier 29 is controlled by the synchronizing impulses.

Assume, for example, that the amplifier 29 starts to drift in a direction such that the plate current of the last tube |0| increases. This causes a decrease in the positive potential at the anode whereby the control grid |21 of the balancer tube |23 becomes more negative. This increase in negative bias is indicated by the dotted line 209. The picture signals and the synchronizing signals are likewise shifted as indicated by their dotted line positions.

The greater the shift of the synchronizing impulses in a negative direction. the less the Value of the plate current pulses appearing in the output circuit of the balancer tube |23. This decrease in plate current pulses is indicated by the dotted line positions of the plate current rectangles shown at 205a. The smaller values of plate current pulses are integrated by the condenser 3| to produce a steady flow of biasing current having a decreased value as indicated by the dotted line 2| I.

It follows that an increase in current in 'the output circuit of the direct-current amplifier 29 causes a decrease in the negative bias applied to the input circuit thereof, and, since this decrease in negative bias causes a decrease in plate current in the amplifier output circuit, a balance is effected. In other words, before the output current of the amplifier 29 can either increase or decrease any appreciable amount, the bias on the first tube 9| of the amplifier is changed to counteract the change in output current.

The completeness of balance or the percentage of drifting which is balanced out is proportional to the gain around the closed loop from the balancer control grid |21 through the amplifier 29 and back to the balancer control grid |21. For example, considering the circuit shown in Figure 4, if the gain around the loop is 10,000 (considering included the effect of having the balancer tube |23 operative only a small percentage of the time), and the plate voltage of the amplifier tube 9| varies or drifts one volt, a change in voltage on the grid |05 of the second amplifier tube |0| of one ten-thousandth Volt would be enough to cancel out the one volt change completely. However, as this action cannot take place, the new equilibrium point is something less than one ten-thousandth volt different on the grid |05 of the second amplifier tube than it was before the one volt drift occurred. The balance action is then more than 99.99% complete.

The highest frequency of noise that may be balanced out is determined by the capacity of the con-denser 3| and by the period of the synchronizing impulses. Noise of near or about the synchronizing frequency cannot be balanced out because it may happen between successive synchronizing impulses. Nothing is gained, therefore, by decreasing the capacity of the condenser beyond a certain region because no more of the noise spectrum can be eliminated and, as the condenser gets smaller, a larger modulation of the picture background results. That is, if the capacity of the condenser 3| is too small, or if the resistance of the plate resistor in shunt thereto is too small, the value of the bias on the control grid 95 of the amplifier tube 9| Will change appreciably between synchronizing impulses. 'I'he condenser 3| should have sufficient capacity to prevent any appreciable modulation of the picture background. On the other hand, it should not be so large that the balancer responds too slowly to changes in the condition of the amplifier, since when drifting occurs it should be corrected immediately.

From the above remarks it will be appreciated that the capacity of the condenser 3| and the resistance of the plate resistor |31 may have values varying within, wide limits. As an example of suitable values, however, a condenser having a capacity of one microfarad and a resistor having a resistance of 100,000 ohms have been found satisfactory.

Referring to- Figure 6, Where parts similar to those in Figure 1 are indicated by like reference numerals, a circuit is shown in which the synchronizing impulses are not transmitted through the high gain direct-current amplifier 29. In-

` stead, they are amplified to the necessary value in the synchronizing amplifier 43, and supplied in proper phase through a potentiometer 2|5 to the transmission line |-3.

'Ihe synchronizing impulses are supplied to the input circuit of the balancing amplifier 33 through a conductor 2|1 for the purpose of controlling the balancing circuit in the manner described above.

An amplifier 2|9 comprising two electric discharge tubes connected in cascade is provided to prevent the synchronizing impulses which are supplied to the balancer amplifier 33 through the conductor 2|1 from being impresse-d upon the transmission line |-3. It will be noted that this permits the application` of balancing impulses and synchronizing impulses to the amplifier 33 and the line |-3, respectively, having different amplitudes whereby the control of the system is more flexible. If desired, the blocking amplifier may consist of an odd number of tubes, but in that case the number of tubes in either the amplifier 29 or in the balancer circuit 33must be changed in order to feed energy from the output of the amplifier 29 to its input in the proper phase to oppose changes in the output current.

From an inspection of Figure 6, it will be apparent that, if it is desired to transmit picture signals free from synchronizing impulses, the connection through the potentiometer 2|5 may be omitted. In this case the impulses which are supplied from the synchronizing amplifier 43 serve only to prevent drifting of the direct-current amplifier. This modification is of particular Value where the amplifier is utilized for purposes other than the amplification of picture signals.

It will also be evident to those skilled in the art that various eXpedients may be employed for interrupting signal voltages periodically and supplying control impulses of the opposite polarity during the periods these signals are interrupted.

An example 'of such an expedient will now be described and although it, like the preceding embodiment of the invention, is described in connection with the transmission of pictures, it should be understood that my improved directcurrent amplifier is of general application.

A separate generator for the synchronizing or balancing impulses may be omitted by giving the picture a steady component of light of such value that the variations due to they picture never modulate the light more than a certain percentage. A picture signal having this character is shown in Figure 7. It will be noted that the picture signal is always above the dotted line axis 22|, While during the interval between line scanning the signal drops to a zero value. With respect to the dotted line axis 22|, this zero value of current corresponds to a current impulse having a polarity opposed to that of the signal current and, therefore, it may be utilized for synchronizing and for controlling the balancer circuit 33 in the manner previously described.

. One form of apparatus for obtaining the type of picture signal shown in Figure 7 is illustrated in Figures 8 and 9, where parts similar to those in Figure 1 are indicated by like reference numerals.

Referring to Figures 8 and 9, the steady component of light is supplied to the photoelectric cell I9 b-y means of a half-silvered mirror 223 positioned between the scanning disc l and the iilm l5. A portion of the light passes through the half-silvered mirror 223, While another portion is reflected to a mirror 225 and, from the mirror 225, to the photoeleotric cell I9.

As shown in Fig. 10, the synchronizing holes are preferably omitted from the scanning disc 'l when employing the half-silvered mirror 223 since they would have no function, the mask 221 being of the proper dimensions to provide a synchronizing interval during which no light passes through the scanning disc.

Although I have described the preferred embodiment of my invention in which the picture signals and the synchronizing signals are of opposite polarity, it should be understood that they may be of the Same polarity providing the synchronizing signals are given an amplitude considerably greater than the amplitude of the picture signals.

For example, referring to Fig. 5, if the picture signals are given a positive polarity at the balancer tube |23 and an amplitude which is approximately one-half of the amplitude of the synchronizing signals, it will be apparent that only the synchronizing impulses will produce a flow of current in the plate circuit of the balancer tube. Signals of this character can be obtained by providing the proper number of. amplifier stages in the picture and synchronizing channels in accordance with well known practice.

From the foregoing description, it will be understood that various modifications may be made in my invention without departing from the spirit and scope thereof, and I desire therefore that only such limitations shall be placed thereon as are necessitated by the prior art and are set forth in the appended claims.

I claim as. my invention:

l. The method of operating a direct-current amplifier for amplifying electrical energy which comprises impressing said energy upon the input circuit of said amplifier periodically whereby there are intervals during which said energy is not being impressed thereon, impressing a voltage impulse during and only during said intervals for controlling a bias voltage in said amplifier.

3. The method of operating a direct-current amplifier which comprises impressing thereon periodically signal energy to be amplified whereby there are blank periods during which said signal energy is not being impressed upon said amplifier, and app-lying biasing venergy to said amplifier during said blank periods, said energy having a value dependent upon the condition of said amplifier during said blank periods.

4. rl`he method of operating an amplifier for amplifying electrical signals which comprises impressing said signals upon the input circuit of. said amplifier periodically, whereby there are intervals during which said signals are not impressed upon said input circuit, impressing a control voltage upon said amplifier during each of said intervals to produce an amplified energy impulse, storing up each, of said amplified impulses in accordance with the condition of said i amplifier during the preceding signal period, and controlling the condition of said amplifier during the succeeding signal period in accordance with said stored energy.

5. In a picture transmitting system including a direct current amplifier, the method of transmitting pictures which comprises transmitting picture signals corresponding to scanning lines, transmitting synchronizing signals between scanning lines, impressing saidsignals upon said direct-current amplifier, amplifying said synchronizing signals in accordance with the directcurrent component appearing in the output of said amplifier, and applying a control voltag-e to said amplifier having a value which depends upon said amplified synchronizing signals and having the proper polarity to oppose any change in said direct-current component.

6. In combination, a direct-current amplifier having an input circuit and an output circuit, a second direct-current amplifier means for so coupling said outp-ut circuit to said input circuit through said second amplifier that a degenerative loop circuit is formed, means for impressing signal voltages at one time and control voltages at a different time upon said input circuit, and means for permitting only said control voltages to passv through said loop circuit.

'7. In combination, a direct-current amplifier having an input circuit and an output circuit, a second direct-current amplifier, means for so coup-ling said output circuit to said input circuit through said second amplifier that a degenerative loop circuit is formed, means for impressing signal voltages and control voltages alternately upon said input circuit, and means for permitting only said control voltages to pass through said loop circuit.

8. In combination, a direct-current amplifier having an input circuit and an output circuit, a

second direct-current amplifier, means -for so coupling said output circuit to said input circuit through said second amplifier that a degenerative loop circuit is formed, means for impressing signal voltages and control voltages alternately upon said input circuit, means for preventing the circulation of said signal voltages in said loop circuit, and means for controlling a bias voltage in said amplier in accordance with said control voltages.

9. Apparatus according to claim 8 characterized in that said coupling means includes an electric discharge tube having a bias voltage which varies in accordance with the direct-current component of the output of said amplier.

10. A direct-current amplier having an input circuit and an output circuit, feed-back means for feeding energy from said output circuit toi said input circuit in the proper phase to oppose any change in the output of said amplifier, means for impressing signals upon said input circuit periodically, and means for making said feedback circuit inactive during the periods said signals are being impressed upon said input circuit and for making it active duiing the intervals between said signal periods.

11. Apparatus for amplifying signal voltages, said apparatus comprising a direct-current amplifier having an input circuit and an output circuit, a feed-back circuit coupling said output circuit to said input circuit, said feed-back circuit including an electric discharge tube biased to the region of cut-off and being connected to oppose any change in current in said output circuit, means for impressing said signal voltages periodically upon said input circuit whereby there are intervals of no signal, means for impressing voltage pulses upon said input circuit during said intervals, said pulses having the proper polarity to cause a positive voltage pulse upon, the input electrodes of sai-d electric discharge tube.

12. Electrical apparatus comprising an amplifier having an input circuit, means for impressing signal voltages upon said input circuit periodically whereby there are signal periods separated by intervals of no signal, means for applying a control voltage to said amplier during each of said intervals whereby each control voltage is amplified to a value corresponding to the condition of the amplifier during the interval the said control voltage is applied, means for charging a condenser in accordance with the value of said amplified control voltage, and means for controlling sai-d amplifier in accordance with the charge on said condenser.

13. A direct-current amplifier having an input circuit and an output circuit, means for so coupling said output circuit and said input circuit that a degenerative loop circuit is formed, said coupling means including an electric discharge tube biased beyond cut-off, means for producing signal voltages and control voltages of opposite polarity and for impressing them alternately upon said input circuit, said loop circuit being so designed that said control voltages are of positive polarity when impressed upon said electric discharge tube whereby they are passed through said loop circuit, means responsive to a change in the output of said amplier for changing the bias of sai-d electric discharge tube whereby pulses of control current appear in the output circuit of said tube having values depending upon said change in bias, and means including a condenser shunted by a resistance unit for applying a controlling voltage to said direct-current amplier, said coupling means being connected to charge said condenser in accordance with said pulses of control current.

14. The method of maintaining the amplification factor of a direct-current amplier substantially constant which comprises supplying signals to the amplifier periodically whereby there are intervals during which said signals are not supplied thereto, supplying control signals to the amplifier during said intervals, deriving from the amplied control signals a unidirectional potential and utilizing said potential for controlling the gain in the amplifier.

ARTHUR W. VANCE. 

